The present invention generally relates to image forming apparatuses and methods of controlling an output of a semiconductor laser of the image forming apparatus, and more particularly to an image forming apparatus such as a digital copying machine, a laser printer and a facsimile machine which forms an image by use of a semiconductor laser beam and a method of controlling the output of the semiconductor laser of such an image forming apparatus.
Generally, an image forming apparatus such as a laser beam printer is provided with a photodetector for detecting a laser power (optical output) of a semiconductor laser, and a current injected to the semiconductor laser from a constant current source is variably controlled responsive to an output signal of the photodetector so as to control a power of the laser beam constant. The construction of the photodetector may be simplified by use of a photodiode provided within a semiconductor laser package.
In the present specification, an operation of controlling the power of the laser beam constant by variably controlling the current injected to the semiconductor laser depending on the output signal of the photodetector will be referred to as a "power check".
Generally, the power check is carried out by turning ON the semiconductor laser for a certain short and constant time when no image formation takes place. This certain short and constant time differs depending on the image forming apparatus. In other words, the power check is carried out during a time corresponding to a no-image region in each main scanning line or during a time corresponding to a region outside an image region between two successive image formations.
However, depending on the circuit construction, response speed and the like of a feedback control system which detects the laser power of the semiconductor laser and controls the power of the laser beam constant responsive to the detected laser power, the semiconductor laser may remain turned ON for the power check even after the scanning of a photosensitive body starts. In addition, the semiconductor laser may remain turned ON for the power check even after the scanning of the photosensitive body starts also depending on an effective scan time ratio of an optical system between a time in which the image region is scanned and a time in which one line is scanned. Especially in the case where the power check is carried out responsive to a timing signal which is generated at constant intervals and a supply of the timing signal to the feedback control system is permitted when the power check takes place outside the image region, the power check is carried out in a waiting mode in which the photosensitive body is stationary and no image formation takes place with a frequency identical to a frequency with which the power check is carried out in an image formation mode in which the image formation takes place. For example, the image formation mode corresponds to a copy mode in the case of a copying apparatus and corresponds to a print mode in the case of a printer. But when the power check is carried out while the photosensitive body is stationary, the laser beam continuously hits the same portion on the photosensitive body and this portion of the photosensitive body becomes deteriorated. As a result, the picture quality of the image formed in the image forming apparatus becomes deteriorated due to the deteriorated portion of the photosensitive body, and such a deterioration in the picture quality is observed as a white line or a black line depending on a developing system employed in the image forming apparatus. It is highly desirable to prevent such a deterioration in the picture quality because one of the outstanding features of the image forming apparatus such as the laser printer is the extremely fine picture quality that is obtainable due to the use of the semiconductor laser.
Next, a description will be given of a conventional image forming apparatus which carries out the power check for every main scanning line. In such an image forming apparatus, a line synchronizing signal and an image region signal are successively outputted for every line, and the power check is carried out before each line synchronizing signal is outputted. The image region signal indicates a region in which the image exists. But when an attempt is made to form an image having a high picture quality at a high speed, a duration of one line (that is, an interval with which the line synchronizing signals are outputted) becomes short and it becomes necessary to set a frame frequency to a high frequency. In order to avoid setting the frame frequency to the high frequency, a lens and a deflector of the image forming apparatus must be designed to set the scan time ratio to a large value, however, this would make a time in which the power check is carried out within one line scanning period extremely short. In actual practice, it is virtually impossible to control with a high accuracy the optical output of the semiconductor laser within such an extremely short time.
Therefore, there is a demand for an image forming apparatus which forms an image having a high picture quality and positively carries out a power check which guarantees the high picture quality of the formed image.